Optical devices are widely used in sensing, monitoring, control and communication systems. A typical optical system comprises an optical transmitter and an optical receiver. An optical receiver usually comprises a photo-detector which converts incoming optical signals into electrical output signals for processing by downstream signal processing circuitry. A photo-detector usually comprises a semi-conductor absorbing layer. When optical signals of an appropriate wavelength impinge on the absorbing layer of a photo-detector, electron-hole pairs will be created. Bias-voltage at the terminals of the photo-detector will accelerate the carriers in the electric field between the terminals, whereby in-coming light is converted into electric current. A photo-detector may comprise a photo-diode, for example, a PIN diode for an avalanche diode, photo-resistors or, more recently, MSM photo-detectors. Photo-detectors are typically at a pre-determined biasing condition adapted for specific applications.